Article pubs.acs.org/JPCC
Solvent Controlled Energy Transfer Processes in TriarylamineTriazole Based Dendrimers Fabian Zieschang,† Alexander Schmiedel,† Marco Holzapfel,† Kay Ansorg,‡ Bernd Engels,‡ and Christoph Lambert*,† †
Institut für Organische Chemie, Wilhelm Conrad Röntgen Research Center for Complex Material Systems, Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany ‡ Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S Supporting Information *
ABSTRACT: Fluorescence upconversion measurements of three different dendrimers G1−G3 based on triarylamines connected by triazole linkers show a strong and fast initial decay of fluorescence anisotropy for t < 2 ps followed by anisotropy decay on a much longer time scale (10−100 ps). At the same time, a pronounced solvent relaxation takes place. Comparison of the decay data in different solvents revealed that the initial decay of fluorescence anisotropy is governed by a competition of solvent relaxation and incoherent hopping of energy between the different dendrimer branches. Thus, it is decisive to discriminate between energy transfer processes in the Franck−Condon state or in the solvent relaxed state. We demonstrate that even for charge transfer chromophores, where a large Stokes shift leads to very weak spectral overlap of donor fluorescence and acceptor absorption, rapid homotransfer is possible if there is sufficient spectral overlap with the time zero fluorescence spectrum.
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INTRODUCTION In this paper we investigate energy transfer processes in a series of triarylamine based dendrimers of different size. Energy and electron transfer are both processes that play a fundamental role in organic materials for optoelectronic devices and in artificial systems mimicking natural photosynthesis.1−5 Triarylamines (TAA) are one of the key players for hole transfer applications in e.g. OLEDs but also in solar cells and even in organic field effect transistors.6−9 Their charge transport properties have thus extensively been investigated. One way to ensure morphological stability of materials is to make molecules that, owing to their molecular shape, entangle and show high glass transition temperatures.6,10 Monodisperse dendrimers may fulfill this requirement. Hartwig et al. and others synthesized dendrimers based on phenylene diamines and benzidine moieties and investigated their redox properties.11−19 Besides their charge transfer properties, also the energy transfer properties,20−35 two-photon absorption,33,36 and sensing applications34,35 of those and similar amine based dendrimers have been in focus of several investigations. Concerning energy transfer issues of hyperbranched triarylamine dendrimers, Goodson at al.23 observed a fast initial decay (
